Electrodialysis device having tapered gasket thickness



Dec. 19, v1967 WILLIAM KWO-WE| CHEN 3,359,196

ERED GASKET THICKNESS ELECTRODIALYSIS DEVICE HAVING TAP l Filed May 22 6Sheets-Sheet l C'rigina INVENTOR. WILLIAM KWO-WE| CHEN vATTORNEY mQorOSi m @2m Dec. 19, 1967 WILLIAM KWo-WEI CHEN 3,359,196v

ELECTRODIALYSIS DEVICE HAVING TAPERED GASKET THICKNESS Original FiledMay 22, 1959 6 Sheets-Sheet 2 INVENTOR. WILLIAM KWO-WEI CHEN @mw/2W lATTORNEY Dec. 19, 1967 WILLIAM Kwo-WEI CHEN 3,359,196

LECTRODIALYSIS DEVICE HAVING TAPERED GASKET THICKNESS Original Filed May22, 1959 6 Sheets-Sheelr 5 FIG. 3

zaza D|LUTE 105@ CONCENTRATE INVENTOR. WILLIAM KWO-WEI CHEN 4M ,4pm

' ATTORNEY 6 Sheets-Sheet 4 ATTORNEY WILLIAM Kwo-w5| CHENELECTRODIALYSIS DEVICE HAVIN" 4 plu. Ll www@ wfff@ @555% @w M J j J 9j Il i] [95 I I Z j] j j [to l S I l XM \N l E ew M @ie @M7oW/d o o o @le Mw@ M @Mfg/V O M o oy/,WIG D Ra/e 5 6 7, j@ M/e oo- @Hw/o, O o Q// E Uff@ n M O l 7/0 A RT a a, i f5 U0 Y a fz zz af 72j/ iz of 2 U07 mm@ @w7M ZZ/i Za/ M M w C Dec. 19, 1967 Original Filed May 22, 1959 DeC- 19,1967v WILLIAM KWo-WEI CHEN 3,359,195 V vELECTRODIALYSIS DEVICE HAVINGTAPERED GASKET THICKNESS Criglnal Filed May 22, 1959 6 Sheets-Sheet FIG.5v

NVENTOR. A

. 4 l WILLIAM KWO-WEI CHEN l Y BY M4- 4?@ 1 ATTORNEY Dec. 19, 1967WILLIAM KWG-WE1 CHI-:N 3,359,196

ELECTRODIALYSIS DEVICE HAVING TAPEHED GASKET THICKNESS Origlnal FiledMay 22, 1959 6 Sheefcs-Sheet e FIG. e 'F'G-gz 4 Co NTRATE 10J /a/CONCENTRATEJJC INVENTOR. WILL IAM KWO-WEI CHEN ATTORNEY United StatesPatent Ofiice Patented Dec. 19, 1967 3,359,196 .ELECTRODIALYSS DEVICEHAVING TAPERED GASKET THICKNESS William Kwo-Wei Chen, Stamford, Conn.,assignor to American Machine & Foundry Company, a corporation of NewIlersey Application Dec. 6, 1962, Ser. No. 243,217, now Patent No.3,223,696, which is a continuation of applications Ser. No. 815,188, May22, 1959, and Ser. No. 195,352, Mar. 38, 1962, now Patent N 3,228,867,dated Jan. 11, 1966; said Ser. No. 195,352, being in turn, a division ofSer. No. 815,188. Divided and this application July 19, 1965, Ser. No.485,662

2 Claims. (Cl. 2114-301) This application is a division of applicationSer. No. 243,217 iiled Dec. 6, 1962, now U.S. Patent No. 3,223,- 606which is a continuation of my copending application S.N. 815,188 tiledMay 22, 1959, now abandoned, and Serial No. 195,352 led Mar. 30, 1962,now U.S. Patent No. 3,228,867, as a division of application S.N.815,188.

This invention relates to multiple chamber electrodialysis devices andmethods of operation of these devices. More particularly, the inventionrelates to electrodialysis with pennselective membranes and to thehydraulic and electrical arrangement of such membranes inelectrodialysis devices.

In the operation of multicell electrodialysis apparatus, concentrationpolarization and consequent water dissociation at the membrane surfacesdue to electrolysis eiects have been diicult practical problems.Concentration polarization results in lower current eilciency and alsocauses pH changes in the solutions undergoing electrodialysis. Such pHchanges may also induce precipitation of pH sensitive salts such ascalcium carbonate, which, in turn, would cause scaling of membranes andobstruction of ow channels. High linear velocity of solutions has beenused to reduce this diculty. However, substantial residence time of thesolution in contact with the membrane surface is desirable to achieve aneconomical degree of demineralization.

Various designs and methods have been proposed to combine both prolongedcontact of solution with ion exchange material and rapid solutioncirculation. One approach has been to recirculate product streams withthe attendant disadvantage of extra recirculation pumps, valves, storagetanks and control devices. This method is described in British PatentNo. 682,703.

Another approach to minimizing concentration polarization has been theuse of tortuous hydraulic channels through the intermembrane spacers asdescribed in U.S. Patent No. 2,708,658. This complex spacer designgenerally results in the ineilcient use of membrane surface areas andhigh pumping energy requirements, and since the flow through thesetortuous channels is in series, plugging at any point in the flow pathwould seriously reduce the rate of ilow. The high pressures required forthe operation of such a system also require the use of very rigidmembranes to prevent their distortion under unbalanced pressureconditions.

Another method of dealing with polarization problems has been proposedin U.S. Patent No. 2,694,680. A number of independent electrodialysisassemblies are arranged in a staggered system of stages to achieveincremental demineralization from stage to stage. Solution velocitiesand electric current ilow are adjusted in each stage for greatesetlciency. However, the use of individual electrodes, different directcurrent voltages and separate packaging leads to engineeringcomplications and expensive systems. For small electrodialysis systemsin particular, where the flow rates are less than about two gallons perminute, it would be very impractical to provide separate end blocks, endplates, electrodes and interconnecting piping ysystems for each smallgroup of cells. Separate electrode streams for each assembly would beespecially impractical for low capacity systems because they requirespecial handling.

Moreover, in Australian Patent No. 164,040 an electrodialysis process isdescribed in which the stream to be demineralized is passed successivelyin series through every alternate compartment from one electrode toanother to achieve a degree of demineralization higher than would resultfrom parallel rather than series flow. The maximum electrical currentthat can be used in an electrodialysis cell without excessiveconcentration polarization is proportional to the concentration ofelectrolytes n the cell. In a series stack the limiting current throughthe stack is therefore governed by the concentration in the last productcell. This requirement automatically irnplies that the current flowingthrough the preceding product cells is lower than the maximum currentwhich may be applied to these cells, and the process results in theuneconomical use of these cells. Since all compartments are in series,high hydraulic pressure requirements are also encountered at desirablesolution velocities.

Therefore, it is an object of this invention to provide a method ofelectrodialysis which controls concentration polarization effects.

It is also an object of this invention to provide an electrodialysisdevice which incorporates means for producing high solution velocitiesin the sections of an electrodialysis device where such high velocitiesare required to reduce concentration polarization while maintaininglower solution velocities where maximum residence time (for higherdegrees of demineralization) and minimum hydraulic pressure drops aredesired.

It is also an object of this invention to provide a multicellelectrodialysis device which produces the desired gradation in solutionvelocity and reduces concentration polarization by incorporating aseries of cascaded' membrane cells between a sin-gle electrode pair sothat the cross sectional area in succeeding cascades progressivelydecreases in the direction of flow.

It is also an object of this invention to provide a method ofelectrodialysis by which a solution can be demineralized with a minimumexpenditure of hydraulic pumping energy and a minimum degree ofpolarization between a single pair of electrodes.

It is also an object of this invention to provide a method of fluiddemineralization in a multicompartment electrodialysis devicecharacterized by the step of establishing a solution velocity gradientin inverse relation to the solution salinity gradient at constantcurrent density between a single pair of electrodes.

It is also an object of this invention to provide a method of iluiddemineralization in a multicompartment electrodialysis devicecharacterized by the step of establishing between a :single pair ofelectrodes a current density gradient in direct relation to a solutionsalinity gradient at constant solution velocity.

It is also an object of this invention `to provide a method of fluiddemineralization in a multicompartment electrodialysis devicecharacterized by the step of establishing between a single pair ofelectrodes a graduated membrane area increasing in size in the directionof fluid flow.

These and other objects of the invention are described in the followingdetailed account of the invention and in the attached drawing in whichreference characters of similar elements correspond and in which:

FIG. 1 is a schematic cross-section of an electrodialysis stack, thepreferred form of the invention, showing alternate anion and cationpermeable membranes arranged to patterns or hydraulic circuits aredepicted by lines with arrows;

FIG. 2 is an exploded isometric view of the electrodialysis stack shownschematically in FIG. l;

FIG. 3 is an exploded isometric view of a modified form of theinvention, an electrodialysis stack employing membranes andintermembrane gaskets having a triangular cross-section;

FIG. 4 is an exploded isometric view of another form of the invention,an electrodialysis stack employing membranes and intermembrane gasketsof hexag-onal crosssection and utilizing a radial flow pattern;

FIG. 5 is an exploded isometric view of an electrodialysis stackemploying wedge-shaped intermembrane gaskets, another form of theinvention;

FIG. 6 is a schematic cross-section of an electrodialysis stackemploying intermembrane gaskets of varying thicknesses, another form ofthe invention;

FIG. 7 is an isometric view of an assembled electrodialysis stackemploying membranes and intermembrane gaskets of varying cross-sectionalarea, another form of the invention;

FIG. 8 is a schematic cross-section of the electrodialysis stack shownin FIG. 7.

In general the various embodiments of the invention include a method ofelectrodialysis in which the velocity of liquid or the density ofelectric current flowing through an electrodialysis stack is adjustedalong the solution flow path in relation to polarizing conditionsestablished by a salinity gradient between the electrodes in theelectrodialysis stack configurations in which these methods may beapplied.

When raw feed solution enters the electrodialysis device, it is dividedinto two main parts. The rst part becomes the product or dilute streamand the second part becomes the waste or concentrate stream. Thesedescriptions apply when dilution rather than concentration is thedesired effect.

As the dilute stream becomes more dilute in the course of tiow throughthe device, a salinity gradient is established between the beginning andend of the dilute flow. Near the end of the dilute flow, as the streamis about to emerge from the device as a demineralized product, thesolution is no longer so good an electrical conductor as in its initialconcentrated form. Polarization, or water dissociation at the membranesurfaces, as a result of too dilute (and therefore poorly conducting) alayer forming at the interfaces becomes particularly acute as the ratioof current density to salinity increases.

Increased solution velocity in relation to the decrease of salinitytends to overcome polarization, and the method of achieving velocitycontrol is a first feature of this invention.

By decreasing the current density in particular parts of the devicewhere the salinity has decreased, the ratio of current density tosalinity is reduced and polarization is controlled. Reducing the currentdensity as the salinity declines tends to maintain an optimum ratio ofcurrent density to salinity and controls polarization. Reduction ofcurrent density as salinity declines is a second feature of thisinvention.

Both control of fluid flow velocity and change of current density may beused separately or together in the same device in the practice of thisinvention, but for simplicity they are illustrated separately hereinsince both contribute to control of the same kpolarization effect.

According to a preferred form of this invention, as shown in FIG. 1,solution residence time is controlled by connecting groups of cells orcascades hydraulically in series between a single set of electrodes 30,32. By using a single electrode pair, a saving is made in the cost ofsupplying multiple electrodes, and the power losses associated withelectrode over-voltage are reduced.

Each hydraulic pass at right angles to the direction of electricalcurrent as indicated by the arrow 40 becomes an electrical cascade, andthe change in concentration per cascade is proportional to the number ofdiluting cells or chambers per cascade.

Thus, in FIG. l and FIG. 2, the first cascade A containing four dilutingcells (a diluting cell is formed within a stack when it is bounded by ananion membrane on the anode side and a cation membrane on the cathodeside) will have a dilute stream solution velocity only 3A that of thesecond cascade B with only three diluting cells.

Since the change in concentration in each cell is proportional to theelectric current (which is constant in this case) and inverselyproportional to the flow rate in the cell, it is seen that the iirstcascade A will undergo 4/2 the change in concentration of the secondcascade B. Since all the cells are electrically in series, Athe averagecurrent density, CD, in all cells will be the same. In the dilutingcells, however, the average electrolyte concentration will beprogressively lower in each succeeding cascade. The ratio of currentdensity to solution normally, CD/N, is an indication of polarizingconditions and will increase from the rst cascade A to the seventhcascade G. In FIG. l, cascades A, B, C, D, E, F, G contain 4, 3, 3, 2,2, l, l cells respectively.

Since higher CD/N ratios are tolerable at higher solution velocities,successive electrical cascades comprise a progressively reduced numberof diluting cells in parallel. This type of membrane stack configurationor array thereby facilitates the use of designs in which the flow ratesthrough different cells are optimized for the electrochemical conditionswhich exist therein.

FIG. 2 is an exploded isometric view of the electrodialysis stack inFIG. 1 showing the components used in the preferred form of theinvention and the methods used to direct the various streams through thecomponents. The principal components are end blocks 60, 62, electrodes30, 32, intermembrane gaskets 50, 51, 52, 53 and membranes 20, 21, 20A,21A. A clamping device, used to hold these components in liquid-tightrelationship, is not shown.

In operation the solution to be demineralized is fed as a single streamto a manifold hole 70 which is formed by the alignment of symmetricalholes 70 in the various components comprising the stack. Anintermembrane gasket 50 serves both to define ya tiow path 80 for theanode rinsing stream and to split the feed stream l100 into threeauxiliary streams 101, 102, 103. The three-way split is achieved Ibyslits 90, -91 in a gasket 50 which communicates with the flow path 80 inthe intermembrane gasket 50 and manifold hole 71. The function of theanolyte stream 101 is to collect and carry away the products ofelectrolysis at the anode 30. Where this stream leaves the stack, it isgiven the designation 101A in the drawing to denote its change incomposition.

The dilute stream 102 undergoes dilution in respect to the solute duringelectrodialysis. It is distributed from a manifold 71 via slits 94 tothe alternately arrayed intermembrane gaskets 52. Where this streamleaves the iiow paths 82 in the gaskets 52 via slits 95, it isdesignated 102A in the drawing to denote the change in composition dueto electrodialysis. In subsequent cascades this stream is designated102B, 102C, 102D, etc., in the drawing to denote progressive additionalchanges in composition.

The concentrate stream 103 undergoes concentration in respect to thesolute during electrodialysis. It is distributed from a manifold 70 tothe alternately arrayed intermembrane gaskets 51 via slits 93. Wherethis stream leaves the fiow paths 141 in intermembrane gaskets 51 viaslits 93, it is designated 103A in the drawing to denote the change incomposition due to electrodialysis. In subsequent cascades this streamis designated 103B, 103C, 103D, etc. in the drawing to denoteprogressive additional changes in composition.

When the rst electrode 30 is an anode and the second electrode 32 is acathode, the first and alternate membranes 20 are cation permselectivemembranes and the intervening membranes 21 are anion permselectivemembranes. In the last parallel pass of each cascade (denoted in FIG. 2as the nth pass of a cascade having n parallel passes) membranes 21A,20A containing only three holes each are used in place of the anion andcation membranes 21, 20. Hydraulic blocks 70, 71 are thereby created atthe inlet stream manifolds. The dilute stream eflluents 102A fromcascade A are thus recombined in the dilute stream manifold hole 72 andare redistributed through the alternately arrayed gaskets 52 in cascadeB. Since cascade B contains fewer parallel passes and a smaller totalcross-sectional flow path for the dilute stream 102A than does cascadeA, the average linear solution velocity will be greater in cascade B. Itshould also ybe noted that the direction of flow of the dilute stream isreversed by this process.

The concentrate stream effluents 103A yare similarly recombined in theconcentrate stream manifold hole 73 and are redistributed through thealternately arrayed gaskets 51 in cascade B. Since cascade B containsfewer parallel passes for the concentrate stream 103A than does cascadeA, the average linear solution velocity will be greater in cascade B.

In the case of the concentrate stream progressively increasing solutionvelocity is not an essential feature. It is shown only for the sake ofsymmetry as -an aid in the description of a cascaded structure.Similarly, the internal development of several process streams 101, 102,103 from a single feed stream is not essential to the practice of thisinvention but shows the preferred method of construction.

The anolyte stream 101A owing out of the top of the stack through holes74 in the anode `and end block respectively is utilized again at thebottom of the stack where it is directed through holes 75 in the endblock 62 and cathode 32 respectively into the flow path 83 of theintermembrane gasket 53. Since the intermembrane gasket 53 has no slitscommunicating with the manifold holes 70, 71, 72, 73, the catholytestream 101A is isolated from the dilute and concentrate streams andleaves the stack through holes 76 in the cathode and end yblockrespectively as strea-m 101B. Alternately, raw feed solution from stream100, the concentrate stream effluent 103G or a portion of the dilutestream efliuent 102G could be used for the cathode wash stream 101A-B.

FIG. 3 is an exploded isometric view of another embodiment of thisinvention in which the end blocks 160, 162, electrodes 130, 132, theintermembrane gaskets 150, 151, 152, 153 and membranes 120, 121 are allof triangular shape. A clamping device, used to hold these components inliquid-tight relationship, is not shown.

In operation the solution to be deminerialized is fed as a single stream100 to the manifold hole 70 at the -base of the triangle which is formedby the alignment of symmetrical holes 70 in the various componentscomprising the stack.

The intermembrane gasket 150 serves both to define a ow path 180 for theanode rinsing stream and to split the feed stream 100 into threeauxiliary streams 1011, 102, 103. The three-Way split is achievedthrough the use of slits 90, 91 in the inter-membrane gasket 150 whichcommunicate with the ow path 180 in the intermembrane gasket 150 and thedilute inlet manifold hole 71. The anode rinse stream 1 is called theanolyte. Its function is to collect and carry away the products ofelectrolysis at the anode 130. Where this stream leaves the stack, it isgiven the designation 101A to denote its change in composition.

The diluterstream 102 is distributed from the dilute stream manifold 71via slits 94 to the alternately yarrayed intermembrane gaskets 152 andis electrodialyzed in the same manner as that described for FIG. 1. Inthis case, however, all intermembrane gaskets 152 Iare connectedhydraulically in parallel, and the desired increase in solution velocityalong the salinity gradient and desalination flow path 182 is achievedby the progressively decreasing cross-sectional flow area in the owpath.

The concentrate stream 103 is distributed from the concentrate streammanifold 70 via slits 92 to the alternately tarrayed intermembranegaskets 151 and undergoes electrodialytic concentration along its owpaths 181. Parallel construction and parallel flow of the concentratestream is illustrated for simplicity but is not essential for theoper-ation of the apparatus.

The electrode rinse stream 101-101A-101B Hows in series through theanolyte ow path 180 and the catholyte flow path 183 in a manner similarto that described for FIG. 2.

Solution entering at the base of the triangle Hows toward the apex.Thus, the flow path becomes constricted as solution ows toward thetri-angle apex, and velocity increases. The increase in velocitycorresponds with a decrease in solution salinity. Decreased salinity isusually conducive to polarization problems, whereas increased velocityof flow has a correcting effect. As the polarization conditions becomemore acute, they are compensated by higher ow rates. The triangular owpath has been chosen as representative of the principle of thisembodiment, but other ow path cross-sections such as trapezoids wouldserve as well, provided only that the solution is forced to increase itsvelocity progressively along the direction of ow.

FIG. 4 is an exploded isometric View of another embodiment of thisinvention in which end blocks 260, 262, electrodes 230, 232,interrnembrane gaskets 250, 251, 252,

, 253 and membranes 220, 221 are all of hexagonal shape.

A clamping device, used to hold these components in liquid-tightrelationship, is not shown.

The functions of the various components are identical with correspondingcomponents shown in FIG. 3. In this case, however, the desired increasein solution velocity along the desalination ow path 282 is achieved byradial ow of the dilute stream 102 from the multiple dilute stream inletmanifolds 70 located in the periphery of the intermembrane gaskets 252toward the dilute stream outlet manifold 72 loc-ated in or near thecenter of the intermembrane gaskets 252. Solution velocity increases :asthe liquid moves to the more constricted central ow path.

The concentrate stream 103 is similarly manifolded in alternatelyarrayed gaskets 251, but this construction feature is not essential tothe oper-ation of apparatus.

The hexagonal shape of the various stack components has been chosen forsimplied illustration. Other polygonal, circular or elliptical shapeshaving a radial symmetry would serve as well, provided only that theflow is directed from the periphery toward the. center or core of thediluting compartments.

FIG. 5 is an exploded isometric view of another embodiment of thisinvention in which intermembrane gaskets 350, 351, 352, 353 arewedge-shaped and so oriented in respect to one another that theircomplete assembly constitutes a right prismatic stack.

The dilute stream 102 is directed through ow path 382 in gaskets 352 sothat the decrease in cross-sectional area in the gasket imparts aprogressively increasing solution velocity along the desalination owpath.

In FIG. 5 the concentrate stream 103 originates outside the stack fromthe feed stream and is directed separately into the concentrate streammanifold 70. The concentrate stream thus flows countercurrent to thedilute stream and is also subjected to progressively increasingvelocity. Countercurrent flow of the concentrate stream is not essentialto the practice of this invention, however, and has been illustratedonly to show its method of application to the invention.

FIG. 6 is a schematic cross-sectional view of an electrodialysis stackdepicting another embodiment of this invention in which theintermembrane gaskets 451, 452 have progressively reduced thicknesses ineach of the successive electrical cascades A, B, C. The dilute stream102 owing in series through each cascade is thereby forced to increasedlinear velocity along the desalination ow path. The concentrate stream103 is similarly affected by the conguration shown in FIG. 6, but thisfeature is not essential to the practice of the invention. The functionof all other corresponding stack components is essentially the same asthat described for FIG. 2.

FIG. 7 is an isometric view of an assembled electrodialysis stackshowing another embodiment of this invention in which the intermembranegaskets 551, 552 have progressively larger areas in each of thesuccessive electrical cascades A, B, C.

The dilute stream 102 and concentrate stream 103 flow paths through thisstack are illustrated by reference to the schematic cross-section shownin FIG. 3. Although the dilute stream 102 solution velocity does notchange throughout the stack array, the total electrical current ow 40 isdistributed over a progressively larger area in the same generaldirection that the dilute stream 102 follows.

The current density (total current divided by the area through which itflows) is progressively, although not necessarily linearly, reducedalong the desalination flow path :and achieves the same depolarizingeffect as that realized by increasing solution velocity at constantcurrent density.

The concentrate stream 103 is shown in parallel and concurrent owrelative to the dilute stream 102, but this feature is not essential tothe application of the invention. All other stack components havefunctions similar to those described for FIG. 2.

Example I An electrodialysis stack constructed as shown in FIGS. l and 2comprised seventeen cation membranes, sixteen anion membranes, sixteendiluting compartments, sixteen concentrating compartments, one anolytecompartment and one catholyte compartment. The diluting andconcentrating compartments arranged in cascades from A to G comprised 4,3, 3, 2, 2, 1 and l compartment each respectively. Each intermembranegasket was ve centimeters wide by thirty-eight centimeters long byone-tenth centimeter thick and had a ilow path area two and one-halfcentimeters by thirty and one-half centimeters (seventysix squarecentimeters). A turbulence promoting screen similar to that shown inU.S. Patent No. 1,972,433 was located within the flow path of eachgasket.

At a pressure of 700 grams per square centimeter and a total ow rate of5 ml./sec to the stack, the flow rates to each stream were distributedas follows:

ML/sec Dilute stream 2 Concentrate Stream 2 Electrode streams (inseries) 1 When the composition of the feed solution was adjusted to3,000 parts per million of sodium chloride at pH 7 and a direct currentpotential of 18 volts applied, a steady state current of 0.55 ampereswas observed. Under these conditions the product water (dilute stream)contained 450 parts per million of sodium chloride and its pH was 6.7.Thus a high degree of dernineralization was achieved with control ofpolarization.

Since slight pH changes normally occur during electrodialysis, the exactpH change at which polarization is considered undesirable is notcritical. A change in pH of about three units in the range of pH 7,however, would be considered unacceptable, whereas a pH change of about0.5 units would be satisfactory in most cases..

Example 1I An electrodialysis stack was'constructed with the componentsused in Example I and comprised sixteen diluting compartments in series,sixteen concentrating compartments in series, one anolyte compartmentand one catholyte compartment. At a pressure of 4.2 kilograms per squarecentimeter and a total ow rate of 5 ml./sec., the flow rates to eachstream were distributed as follows:

Ml./sec Dlute stream 2 Concentrate stream 2 Electrode streams (inseries) l When the composition of the feed solution was adjusted to3,000 parts per million of sodium chloride at pH 7 and a direct currentpotential of 15 volts applied, a steady state current of 0.55 ampereswas observed. Under these conditions the product water (dilute stream)contained 470 parts per million of sodium chloride and the pH was 6.6

Thus it is seen that a considerably higher pressure is required with allthe cells in series to achieve the same degree of demineralization asthat achieved in Example I.

After an overnight operation under the above described conditions, itwas found that this excessively high pressure caused leakage andultimate failure of the system.

There have thus been described methods and apparatus for electrodialysisin which the velocity of liquid and the density of electric currentowing through an electrodialysis system are adjusted along the solutionilow path in relation to polarization conditions and in particular inrelation to the salinity gradient in the stacked membrane and gasketarray, and the ratio of current density to salinity is controlled.

What is claimed is:

1. Inan electrodialysis device, a pair of electrodes and a plurality ofion-permeable membranes and intermembrane gaskets arranged between theelectrodes and providing alternate diluting and concentratingcompartments, each of said compartments being provided with a fluidinlet and outlet, means for connecting the Huid inlets and outlets ofsaid diluting and concentrating compartments, at least some of thegaskets between membrane pairs having a thickness which is progressivelydecreased in the direction of the dilute stream ow and forming dilutingcompartments having a cross-sectional flow area which is progressivelydecreased in a substantially direct path from the inlet to the outlet ofsaid diluting cornpartments.

2. In an electrodialysis device, a pair of electrodes and a plurality ofion-permeable membranes and intermembrane gaskets arranged between theelectrodes and providing alternate diluting and concentratingcompartments, conduit manifold means for connecting the Huid inlets andoutlets of said diluting and concentrating compartments, at least someof the gaskets between membrane pairs having a tapered thickness andforming diluting compartments having a cross-sectional flow area whichis progressively decreased in a substantially direct path from the inletto the outlet of said diluting compartments.

References Cited UNlTED STATES PATENTS Re. 25,265 10/1962 Kollsman204301 2,794,777 6/1957 Pearson 204-301 ROBERT K. MIHALEK, PrimaryExaminer.

E. ZAGARELLA, Assistant Examiner,

1. IN AN ELECTRODIALYSIS DEVICE, A PAIR OF ELECTRODES AND A PLURALITY OFION-PERMEABLE MEMBRANES AND INTERMEMBRANE GSKETS ARRANGED BETWEEN THEELECTRODES AND PROVIDING ALTERNATE DILUTING AND CONCENTRATINGCOMPARTMENTS, EACH OF SAID COMPARTMENTS BEING PROVIDED WITH A FLUIDINLET AND OUTLET, MEANS FOR CONNECTING THE FLUID INLETS AND OUTLETS OFSAID DILUTING AND CONCENTRATING COMPARTMENTS, AT LEAST SOME OF THEGASKETS BETWEEN MEMBRANE